Concrete Calculator: Yards Needed for a Slab

This concrete calculator works out the volume of concrete you need for a slab, footing, or wall in cubic yards. Enter the project’s length, width, and depth in feet (or inches), and it returns the cubic yards required, plus the same volume in cubic feet and cubic meters for easy comparison.

Formula

The calculator uses the standard volume formula for a rectangular prism:

Volume (cubic yards) = (Length × Width × Depth) ÷ 27

Where:

• Length is the slab length in feet (ft)
• Width is the slab width in feet (ft)
• Depth is the slab thickness in inches (in) – converted to feet by dividing by 12
• 27 is the number of cubic feet in one cubic yard (3 ft × 3 ft × 3 ft)

The result is the exact volume of concrete needed before adding any waste or overage. Most ready-mix suppliers sell concrete by the cubic yard, so this is the number you will order.

Worked Example

Using the default values from the calculator above – a slab that is 10 ft long, 10 ft wide, and 4 inches deep:

1. Convert depth to feet: 4 in ÷ 12 = 0.333 ft
2. Calculate volume in cubic feet: 10 ft × 10 ft × 0.333 ft = 33.3 cu ft
3. Convert to cubic yards: 33.3 cu ft ÷ 27 = 1.23 cu yd

The calculator rounds this to 1.23 cubic yards. If you order from a ready-mix plant, you would typically order 1.25 or 1.5 yards to account for spillage and uneven subgrade.

How to Measure Your Project

Accurate measurements are the foundation of a correct concrete order. Follow these steps:

• Use a tape measure and measure the outside dimensions of the form – not the inside of the hole, which may be wider.
• For a slab, measure length and width in feet. For a footing, measure the width and depth of the trench.
• Measure depth at several points – the ground is rarely perfectly level. Use the average depth, not the deepest or shallowest spot.
• If your project is circular (e.g., a pier), measure the diameter in feet and use the formula: Ï€ × (radius)² × depth ÷ 27.
• For irregular shapes, break the area into rectangles, calculate each, and add them together.

Always measure twice. A 1-inch error in depth on a 10×10 ft slab changes the volume by about 0.26 cubic yards – enough to matter on a delivery order.

Ordering Concrete: What You Need to Know

Ready-mix concrete is sold by the cubic yard, and most suppliers have a minimum order – often 1 cubic yard, but some charge a short-load fee below 3 or 4 yards. When you call, you will give the supplier the length, width, and depth of your project, and they will confirm the yardage.

Add a waste factor of 5–10% for most projects. This covers:

• Spillage when the chute is moved
• Uneven subgrade that takes more concrete than expected
• Leftover concrete in the truck drum (usually about 1/2 cubic foot)
• Small mistakes in measurement

For a 1.23 cu yd slab, adding 10% gives 1.35 cu yd – you would order 1.5 yards. It is almost always cheaper to order a little extra than to pay a second delivery fee for a shortfall.

Common Mistakes and How to Avoid Them

Several errors consistently lead to over- or under-ordering concrete:

• Mixing units: Length in feet, width in inches, depth in feet – convert everything to the same unit first. The calculator handles this by taking depth in inches, but if you calculate by hand, be consistent.
• Forgetting the 27: Cubic yards are not the same as cubic feet. A 10×10×4 slab is 33 cu ft, which is only 1.23 cu yd – not 33 yards.
• Measuring the hole, not the form: If you dig a trench and measure the top width, it may be wider than the bottom. Use the width of the form or the average of top and bottom.
• Ignoring the subgrade: If you compact the ground before pouring, the depth increases slightly. Measure after compacting, not before.
• Rounding too early: Round only at the final step. Rounding 1.23 to 1.2 and then adding waste gives a different answer than rounding after adding waste.

When in doubt, round up to the next quarter yard. A 1/4 yard of extra concrete is about 6.75 cu ft – enough to fill a few low spots, and far cheaper than a second delivery.

Cost Considerations (Without Specific Prices)

The calculator gives you volume, which is the main driver of concrete cost. However, the final price depends on several factors you will need to check locally:

• Concrete mix strength: A standard 3,000 psi mix costs less than a 5,000 psi mix. For a patio, 3,000–3,500 psi is typical; for a driveway that sees heavy vehicles, 4,000 psi is common.
• Additives: Fiber reinforcement, air-entraining agents, or accelerators add to the per-yard price.
• Delivery distance: Most suppliers charge a delivery fee based on mileage from the plant.
• Short-load fees: Orders under the minimum yardage incur an extra charge.
• Pumping: If the truck cannot reach the forms, you may need a concrete pump, which is an additional cost.

To estimate cost, multiply the cubic yards (including waste) by the local per-yard price for the mix you need, then add delivery and any fees. Call at least two suppliers for quotes – prices vary by region and season.

Bagged Concrete vs. Ready-Mix

For small projects (under about 1 cubic yard), bagged concrete mix is often more practical than ordering a ready-mix truck. Here is how they compare:

• Bagged mix: Sold in 40–80 lb bags. An 80 lb bag yields about 0.6 cu ft of concrete, so you would need about 45 bags for 1 cubic yard (27 cu ft). For a 10×10×4 slab (1.23 cu yd), that is about 55 bags.
• Ready-mix: Delivered by truck, mixed on site, and poured directly into the forms. Ideal for slabs over 1 cubic yard, where mixing 50+ bags by hand is impractical.
• Cost per yard: Bagged concrete is significantly more expensive per cubic yard than ready-mix, but you avoid delivery fees and minimum orders.

For a 10×10 ft slab, ready-mix is almost always the better choice. For a small stepping stone or fence post, bags are fine.

Frequently Asked Questions

How many 80 lb bags of concrete do I need for 1 yard?

One 80 lb bag of concrete mix yields approximately 0.6 cubic feet when mixed with water. Since 1 cubic yard equals 27 cubic feet, you need 27 ÷ 0.6 = 45 bags. For a 10×10×4 slab (1.23 cu yd), you would need about 55 bags. This is why ready-mix is usually preferred for anything larger than a few square feet.

How much concrete do I need for a 10×10 slab?

For a 10×10 ft slab, the volume depends on thickness. At 4 inches deep (the most common for a patio), you need 1.23 cubic yards. At 6 inches deep, you need 1.85 cubic yards. Add 5–10% for waste, so order 1.5 yards for the 4-inch slab or 2 yards for the 6-inch slab.

What is the difference between cubic feet and cubic yards?

A cubic yard is a cube that is 3 feet long, 3 feet wide, and 3 feet high. It contains 27 cubic feet (3×3×3). Concrete is ordered by the cubic yard, but many DIY calculators give volume in cubic feet first. Always divide cubic feet by 27 to get yards.

How deep should a concrete slab be?

For a residential patio or walkway, 4 inches is standard. For a driveway that supports cars, 4–5 inches is typical; for heavy trucks or RVs, 6 inches or more. Check local building codes, as they may specify minimum thickness for your area and soil conditions.

Can I use this calculator for a circular concrete pad?

No, this calculator is for rectangular slabs. For a circle, use the formula: Volume (cu yd) = (π × radius² × depth in ft) ÷ 27. For example, a 6-ft diameter pad (radius 3 ft) that is 4 inches deep: (3.1416 × 9 × 0.333) ÷ 27 = 0.35 cu yd. You can also use the calculator by entering the length and width as the diameter (which overestimates slightly) – a 6×6 ft square is 1.11 cu yd, while the true circle is 0.35 cu yd, so it is not a good approximation.

Why does my supplier recommend ordering more than the calculator shows?

Suppliers add a waste factor because the subgrade is rarely perfectly flat, forms may flex, and some concrete sticks to the chute and truck drum. A 5–10% overage is standard. If you are pouring against a foundation or in a trench, the walls may absorb some moisture and reduce the effective volume. It is always better to have a little extra than to run short – a second delivery costs far more than the extra concrete.

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in length | Length | ft | 10 | 0
in width | Width | ft | 10 | 0
in depth | Depth | in | 4 | 0
out yards | Concrete needed | cu yd | (length * width * (depth / 12)) / 27 | 2
out feet | Volume | cu ft | length * width * (depth / 12) | 1
out meters | Volume | cu m | (length * width * (depth / 12)) * 0.0283168 | 2